U.S. patent number 9,306,386 [Application Number 14/026,138] was granted by the patent office on 2016-04-05 for electromagnetic dc pulse power system including integrated fault limiter.
This patent grant is currently assigned to RAYTHEON COMPANY. The grantee listed for this patent is Raytheon Company. Invention is credited to Stephen B. Kuznetsov.
United States Patent |
9,306,386 |
Kuznetsov |
April 5, 2016 |
Electromagnetic DC pulse power system including integrated fault
limiter
Abstract
An electromagnetic direct current (DC) power system includes a
plurality of pulse forming networks (PFN) modules. Each pulse
forming network (PFN) module includes a PFN circuit, a fault
current limiting (FCL) circuit and a cooling system. The pulse PFN
circuit is configured to generate a pulsed DC output power. The PFN
circuit further includes at least one energy storage inductor with
primary winding having a primary winding inductance that controls a
primary impedance of the PFN circuit. The FCL circuit includes a
secondary winding that electrically communicates with the primary
winding. The FCL circuit is configured to receive fault energy
existing in the PFN circuit during a fault event. The cooling
system is configured to cool at least one of the primary winding
and the secondary winding, and remove a portion of the fault
energy.
Inventors: |
Kuznetsov; Stephen B.
(Marlborough, MA) |
Applicant: |
Name |
City |
State |
Country |
Type |
Raytheon Company |
Waltham |
MA |
US |
|
|
Assignee: |
RAYTHEON COMPANY (Waltham,
MA)
|
Family
ID: |
51660019 |
Appl.
No.: |
14/026,138 |
Filed: |
September 13, 2013 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20150077893 A1 |
Mar 19, 2015 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H02H
9/02 (20130101); H02M 1/32 (20130101) |
Current International
Class: |
H02H
9/02 (20060101); H02M 1/32 (20070101); H03K
3/37 (20060101) |
Field of
Search: |
;361/93.9 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
International Search Report and Written Opinion; International
Application No. PCT/US2014/055165; International Filing Date: Sep.
11, 2014; Date of Mailing: Feb. 26, 2015; 10 pages. cited by
applicant .
Sarjeant et al., "Transmission Lines and Pulse-Forming Networks",
High Power Electronics, TAB Books, 1989, pp. 117-136. cited by
applicant.
|
Primary Examiner: Tran; Thienvu
Assistant Examiner: Brooks; Angela
Attorney, Agent or Firm: Cantor Colburn LLP
Government Interests
GOVERNMENT CONTRACT
This invention was made with Government support under
N000024-12-C-4223 Technical Instruction #1 awarded by the United
States Navy. The Government has certain rights in the invention.
Claims
What is claimed is:
1. An electromagnetic direct current (DC) pulse power system,
comprising: a plurality of pulse forming networks (PFN) modules,
each PFN module comprising: a pulse forming network (PFN) circuit
configured to generate a pulsed DC output power, the PFN circuit
including at least one energy storage inductor with primary winding
having a primary inductance that controls a primary impedance of
the PFN circuit; a fault current limiting (FCL) circuit including a
secondary winding magnetically coupled with the primary winding and
to exchange or dissipate fault energy existing in the PFN circuit
during a fault event, and an energy storage capacitor and circuit
interruption apparatus.
2. The electromagnetic DC pulse power system of claim 1, further
comprising an internal cooling system configured to cool at least
one of the primary winding and the secondary winding of the energy
storage inductor or magnetic assembly, wherein the internal cooling
system is configured to cool at least one conductor of the FCL
circuit via a two-phase cooling system that includes a
hydofluoroether (HFE) or fluoroketone dielectric fluid to remove a
portion of the fault energy, and to transfer the removed fault
energy to an external heat exchanger.
3. The electromagnetic DC pulse power system of claim 2, wherein
the FCL circuit includes a commutating device configured to
selectively control the primary inductance of the primary winding
at the energy storage inductor based on the fault event.
4. The electromagnetic DC pulse power system of claim 3, wherein
the commutating device is configured to operate in a first mode
that electrically connects first and second connections of the
secondary winding to decrease a first primary inductance, and a
second mode that disconnects the first and second connections to
increase a second primary inductance that is greater than the first
primary inductance.
5. The electromagnetic DC pulse power system of claim 4, wherein
the commutating device initiates the second mode in response to
determining the fault event exists in the PFN module, through
either detection of fault current, or fault power.
6. The electromagnetic DC pulse power system of claim 5, wherein at
least one current sensor included with a respective first FCL
circuit determines the fault event, and initiates the second mode
in commutating devices contained in the secondary circuit of the
energy storage inductor.
7. The electromagnetic DC pulse power system of claim 6, wherein
initiating the second mode in the first and remaining commutating
devices and associated secondary circuitry discharges the fault
energy equally with respect to the plurality of PFN modules.
8. The electromagnetic DC pulse power system of claim 7, wherein
the second mode in each commutating device in a system of multiple
PFN stages is initiated simultaneously in response to determining
the fault event.
9. The electromagnetic DC pulse power system of claim 8, wherein
the second mode in each commutating device in a system of multiple
PFN stages is initiated sequentially with respect to one another in
response to determining the fault event.
10. The electromagnetic DC power system of claim 9, wherein the
cooling system for the energy storage inductor includes a first
internal primary cooling system configured to cool the primary
winding and a secondary cooling system different from the first
cooling system, the secondary cooling system configured to
internally cool the secondary winding, and transfer at least a
portion of the PFN fault energy to an external media, via the
secondary winding as waste heat.
11. A pulse forming network (PFN) module, comprising: a PFN circuit
configured to generate a pulsed DC output power, the PFN circuit
including an energy storage inductor with a primary winding having
a primary inductance that controls a primary impedance of the PFN
circuit; an energy storage capacitor, a circuit interruption
apparatus; and a fault current limiting (FCL) circuit including a
secondary winding configured to magnetically couple with the
primary winding and associated secondary circuit including passive
L and R components and to exchange or dissipate fault energy
existing in the PFN circuit during a fault event.
12. The PFN module of claim 11, wherein the FCL circuit includes a
commutating device configured to selectively control the primary
inductance of the primary winding based on at least one of the
fault energy, fault current and fault power, and includes, and
includes at least one external inductive and at least one resistive
element to assist in dissipating fault energy away from the main
PFN circuits.
13. The PFN module of claim 12, wherein the commutating device is
configured to operate in a first mode that electrically connects
first and second ends of the primary winding to alter a first
primary inductance, and a second mode that disconnects the first
and second ends to alter a second primary inductance that is
greater than the first primary inductance.
14. The PFN module of claim 13, wherein the commutating device in
the secondary circuit initiates the second mode in response to
determining the fault event exists in the PFN module.
15. The PFN module of claim 14, wherein the commutating device is
configured to initiate the second mode in response to a signal
indicating a rate of rise of current (di/dt) flowing through at
least one of the secondary windings and at least one of the primary
windings.
16. The PFN module of claim 15, wherein the primary winding has a
first number of turns to generate a first impedance, and the
secondary winding has a second number of turns greater than the
first numbers of turns to generate a second impedance greater than
the first impedance, the secondary winding being galvanically
isolated from the primary winding to further reduce a fault current
in the PFN.
17. The PFN module of claim 12, further comprising an
auto-transformer configured to provide a two-stage fault-limiting
system, the auto-transformer including at least one
auto-transformer winding having an impedance that is increased
based on operation of an isolated switching device.
18. A method of protecting an electromagnetic DC pulse power system
from fault energy delivered to a pulse forming network (PFN)
circuit during a fault event, the method comprising:
electromagnetically coupling a fault current limiting (FCL) circuit
to the PFN circuit; detecting the fault event in the PFN circuit;
adjusting the secondary impedance of the FCL circuit to control a
primary impedance of the PFN circuit; inserting external energy
dissipating elements into the FCL secondary circuit with a fast
response time by use of commutating devices; and transferring the
fault energy from the PFN circuit to the FCL circuit in response to
controlling the impedance of the PFN circuit.
19. The method of claim 18, wherein the electromagnetically
coupling further comprises: flowing a primary current through a
primary winding of the PFN circuit, the primary winding having a
first inductance to increase overall circuit inductance and
reflected resistance in the PFN circuit; and energizing a secondary
winding of the FCL circuit to the electromagnetic field to induce a
secondary current that flows through the FCL circuit, the secondary
winding having a second inductance that is greater than the first
inductance to cause resistance in the secondary circuit to be
reflected into the primary circuit.
20. The method of claim 19, wherein the adjusting the secondary
impedance further comprises controlling the first and second
connection of the secondary winding based on a magnitude and
duration of the fault event, including short circuiting the first
and second ends of the secondary winding by use of at least one of
an active switching, commutating device, and a non-linear
resistance clamping device.
Description
BACKGROUND
The present disclosure relates to electromagnetic pulse power
systems, and more particularly, to high power electromagnetic
direct current (DC) pulse power systems including a pulse forming
network (PFN).
Electromagnetic DC pulse power systems utilize one or more PFNs to
shape and control discrete quantities of energy which are
characterized by fast rise-times in either voltage, current or
power. Conventional PFNs typically arrange a combination of
inductors and capacitors in a transmission line, i.e., circuit
network, to yield specific source impedances, specific rise times,
and specific fall times. These PFNs typically output high energy
levels. For example, conventional PFNs may output voltage levels
exceeding 100 kilovolts (kV), current levels exceeding 10 mega-amps
(MA), and power levels ranging from 50 megawatts (MW) to 1 terawatt
(TW). If a system fault occurs, such as a short circuit, peak
current levels, for example 10 MA, may be inadvertently delivered
to components of the PFN such that the pulse power system is
damaged beyond repair.
SUMMARY
According to an exemplary embodiment, an electromagnetic direct
current (DC) power system includes a plurality of pulse forming
networks (PFN) modules. Each pulse forming network (PFN) module
includes a PFN circuit, a fault current limiting (FCL) circuit and
a cooling system. The pulse PFN circuit is configured to generate a
pulsed DC output power. The PFN circuit further includes at least
one storage inductor with at least one primary winding having a
primary inductance that controls a primary impedance of the PFN
circuit. Typically the energy storage inductor is connected in
series with the energy storage capacitor and an electronic
commutating device or electromechanical circuit interruption
element. The FCL circuit includes a secondary winding that is
magnetically coupled with the primary winding. The FCL circuit is
configured to receive fault energy existing in the PFN circuit
during a fault event. The cooling system is configured to cool at
least one of the primary winding and the secondary winding. The FCL
secondary winding provides high voltage isolation from the primary
winding to assist with fault reduction and to enhance overall
safety.
According to another exemplary embodiment, a pulse forming network
(PFN) module comprises a PFN circuit configured to generate a
pulsed DC output power. The PFN circuit includes a storage inductor
with a primary winding having a primary inductance that controls a
primary impedance of the PFN circuit. The PFN module further
includes a fault current limiting (FCL) circuit. The FCL circuit
includes a secondary winding configured to magnetically couple with
the primary winding and to receive fault energy existing in the PFN
circuit during a fault event, and to dissipate fault energy.
According to another exemplary embodiment, a method of protecting
an electromagnetic DC power system from fault energy delivered to a
pulse forming network (PFN) circuit during a fault event comprises
electromagnetically coupling a fault current limiting (FCL) circuit
to the PFN circuit. The method further includes detecting the fault
event in the PFN circuit, and adjusting the secondary impedance of
the FCL circuit to control a primary impedance of the PFN circuit.
The method further includes transferring the fault energy from the
PFN circuit to the FCL circuit in response to controlling the
impedance of the PFN circuit, and protecting sensitive circuit
components such as the main switching devices from excessive
current levels.
Additional features are realized through various embodiments
described in the present disclosure. Other embodiments are
described in detail herein and support various features of the
claims. For a better understanding of the embodiments and features,
the following description and accompanying drawings are
provided.
BRIEF DESCRIPTION OF THE DRAWINGS
For a more complete understanding of this disclosure, reference is
now made to the following brief description, taken in connection
with the accompanying drawings and detailed description, wherein
like reference numerals represent like parts:
FIG. 1 is a schematic diagram of an electromagnetic DC pulse power
system, in accordance with an exemplary embodiment;
FIG. 2 is schematic diagram of an electromagnetic DC pulse power
system, in accordance with another exemplary embodiment;
FIG. 3 is schematic diagram of an electromagnetic DC pulse power
system, in accordance with yet another exemplary embodiment;
FIG. 4 is a diagram illustrating a recovery charge vs. rate of
change of current of a solid-state commutating device included in
an electromagnetic DC pulse power system operating in a current
commutating mode;
FIG. 5 is schematic diagram of an electromagnetic DC pulse power
system, in accordance with still another exemplary embodiment;
FIG. 6 is schematic diagram of an electromagnetic DC pulse power
system, in accordance with another exemplary embodiment;
FIG. 7 is a perspective view of a temperature regulating pulse
forming network (TRPFN) module according to an exemplary
embodiment;
FIG. 8 is a cross-sectional view of a temperature regulating pulse
forming network (TRPFN) module according to another exemplary
embodiment;
FIG. 9 is a flow diagram illustrating a method of protecting an
electromagnetic DC pulse power system in response to a fault event
according to an exemplary embodiment; and
FIG. 10 is a flow diagram illustrating a method of protecting an
electromagnetic DC pulse power system in response to a fault event
according to another exemplary embodiment.
DETAILED DESCRIPTION
Referring to FIG. 1, an exemplary embodiment of an electromagnetic
direct current (DC) pulse power system 100 is illustrated. The
electromagnetic DC pulse power system 100 includes one or more PFN
stacks 102. Each PFN stack 102 includes one or more PFN modules
104. Although a single PFN stack 102 is illustrated, the PFN stack
102 may be part of a larger system of identical PFN modules
configured to produce a high-current, fast-pulse output to a common
load. Although the PFN stack 102 illustrated in FIG. 1 includes
four PFN modules 104, the number of PFN modules 104 is not limited
thereto. For example, the PFN stack 102 may include less than two
PFN modules 104 or more than four PFN modules 104.
The PFN stack 102 may generate energy ranging from approximately 1
kilojoule per stack (1 kJ/stack) to approximately 10,000 kJ/stack.
In addition, the exemplary embodiment of FIG. 1 illustrates the PFN
modules 104 arranged in a common PFN stack 102. The PFN modules 104
may be connected according to various electrical schemes. For
example, individual modules of the PFN stack 102 may be
electrically connected in parallel with adjacent PFN modules 104 to
increase overall current output of the complete system to be
additive. In another example, individual PFN modules 104 may be
electrically connected in series with adjacent PFN modules 104 to
increase overall voltage output of the complete system to be
additive. In still another example, individual PFN modules 104 may
be electrically connected in both series and parallel connections
with adjacent PFN modules 104 to increase overall current and
voltage output of the complete system to be additive. In another
example, the PFN stack 102 may include PFN modules 104 from
multiple different PFN stacks 102.
Each PFN module 104 may utilize a power electronic switching system
(not shown) to pulse modulate an input power supplied by an input
power supply (not shown) at a fast pulse rate, for example a
baseline discharge time of approximately 4.0 milliseconds (ms), to
generate high baseline currents and voltages. For example, the PFN
module 104 may generate a baseline PFN current having a current
level of approximately 100 kA root mean square (RMS), for example,
and a corresponding peak current of 140 kA, for example. The
baseline charge voltage of each PFN module 104 may have a voltage
level of approximately 10 kV, for example. Although each PFN module
104 is described herein as having a common pulse rate, it is
appreciated, however, that each PFN module 104 may have its own
distinct and different discharge time.
Each PFN module 104 comprises a PFN circuit 106 and a fault current
limiting (FCL) circuit 108. The PFN circuit 106 is configured to
generate a primary current i.sub.1. The PFN circuit 106 includes a
primary capacitor C.sub.X, a clamp diode D.sub.X, an electronic
discharge switch (e.g., a thyristor switch assembly) S.sub.X, and a
storage inductor primary winding 110. The primary capacitor C.sub.X
is configured to store energy provided by a DC power supply, and
may discharge the energy to primary winding 110 upon closing of the
electronic discharge switch S.sub.X, and the primary winding 110
transfers the pulsed energy to a common load circuit connected to a
positive and negative bus. Switching of each PFN module 104 into a
common load may be achieved using an array of solid-state thyristor
switch assemblies. The thyristor switch assemblies are sequentially
switched in time to shape the output pulse of a respective PFN
module 104 according to a predetermined wave-shape such as a square
or triangular wave, for example. According to at least one
embodiment, the pulsed energy occurs within a half-period of (e.g.,
1.0 ms). The general method described is applicable for
half-periods down to the nano-second range.
The speed at which a PFN module 104 can switch and shape pulses is
largely dependent on the characteristic time-constant of the power
circuitry and how fast a commutating device can deplete or transfer
their reverse recovery charge (Qrr). Reverse recovery refers to the
ability of a device to commutate current and then within
microseconds being able to block cathode to anode voltage again.
The decay rate (e.g., di/dt) of the on-state current provides an
indication of the depletion/transfer of Qrr. Accordingly, reducing
di/dt at or near the zero crossing of current ultimately improves
the ability of the commuting device 114 to commute regular or fault
current.
The FCL circuit 108 may electromagnetically couple with the PFN
circuit 106, while being galvanically isolated therefrom to form an
isolated electrical current loop. The isolated electrical current
loop allows use of external electrical components in the secondary
FCL circuit that enhances control of the inductance of the primary
winding 110 as discussed in greater detail below. By controlling
the inductance the primary winding 110, the impedance of a
respective PFN circuit 106 may controlled to reduce the fault
current flowing through the PFN circuit 106 during a fault event.
For example, the FCL circuits 108 may be configured to reduce both
maximum di/dt and maximum absolute current. Accordingly, the FCL
circuit 108 may prevent damage of components included in the PFN
circuit 106 during a fault event, such as a short circuit, and may
provide an overall reduction in the volume and size of the
electromagnetic DC pulse power system 100. In further regards to
FIG. 1, which includes PFN stacks 102 having a plurality of PFN
modules 104 and therefore a plurality of FCL circuits 108, the
combination of the FCL circuits 108 may assist in distributing
fault energy resulting from an electrical fault equally among the
plurality of PFN modules 104.
The FCL circuit 108 includes a secondary winding 112 and a
commutating device 114. The secondary winding 112 may be a high
voltage winding and may have a number of turns (N.sub.2) that
control the impedance of the winding. The high voltage realized by
the secondary winding 112 may include a voltage level ranging from
approximately 15 kV to approximately 40 kV. The number of turns
(N.sub.2) of the secondary winding 112 may be different from the
number of turns (N.sub.1) of the primary winding 110. By adjusting
the turns of the primary winding 110 and secondary winding 112, the
respective inductances may be adjusted as understood by those
ordinarily skilled in the art. The use of a high impedance
secondary circuit also allows reduction of overall system volume
and weight for a given fault energy capability. Accordingly, an
impedance differential between the PFN circuit 106 and the FCL
circuit 108 may be generated based on the inductances of the
primary winding 110 and secondary winding 112. In at least one
embodiment, N.sub.2 is determined according to a turn ratio with
respect to the primary winding 110. For example, N.sub.2 with
respect to N.sub.1 may be 8 (e.g., N.sub.2:N.sub.1=8). Accordingly,
the impedance of the FCL circuit 108 may be greater than the
impedance of the PFN circuit 106 by a factor of
(N.sub.2/N.sub.1).sup.2 or 64. Thus the commutating switch in the
secondary loop is only exposed to a fraction of the fault current
as seen in the primary switching devices.
The commutating device 114 is interposed between opposing ends of
the secondary winding 112 and is configured selectively to operate
in an enabled mode or a disabled mode based on the existence of a
fault event. The commutating device 114 may include, but is not
limited to, a fuse, a pyrotechnic triggered fuse, a solid-state
switch, and a thyristor. In at least one exemplary embodiment, the
commutating device 114 may include a supplemental current sensor
113 to detect the fault event in current i.sub.21-i.sub.24. The
commutating device 114 illustrated in FIG. 1 may have a voltage
threshold ranging from about 10 kV to about 35 kV, for example. A
fault detecting device, such as a current sensor 113 connected to
the secondary winding may detect the fault event and output a
trigger signal to the commutating device 114 indicating the fault
event as discussed in greater detail below. Also, the electronic
discharge switch S.sub.X and associated current sensor in circuit
106 may output the trigger signal when a current level i.sub.1,
i.sub.2, i.sub.3 or i.sub.4 exceeds a current threshold, for
example. In another embodiment, however, the commutating device 114
may output the trigger based upon a current rate threshold
representative of the rate of rise of current (di/dt) in either the
PFN circuit 106 and/or the FCL circuit 108, as opposed to an
absolute current level threshold.
The commutating device 114 may selectively operate in an enabled
mode and a disabled mode based on the level of current flowing
through the PFN module 104. If the current level is below a current
threshold, the commutating device 114 may operate in the enabled
mode, that is, it remains in a closed state. If the current level
exceeds the current threshold, the disabled mode is initiated.
Accordingly, the commutating device 114 operates in an open state
such that the inductance of the respective primary windings 110 is
increased, and the fault current flowing through the corresponding
PFN module 104 is reduced. The current threshold may be determined
by the commutating device 114, or by a separate current sensing
device that outputs a fault signal to the commutating device 114
indicating the existence of a fault event.
When operating in the enabled mode, the commutating device 114
effectively connects the first and second ends of the secondary
winding 112 together (i.e., short circuits the ends of secondary
winding 112 together). Accordingly, the effective inductance
transferred to the respective primary winding 110 is reduced as
understood by those ordinarily skilled in the art. Therefore, when
no fault events exist the commutating devices 114 operate in the
enabled mode to electrically connect together the ends of the
respective secondary winding 112, such that the primary windings
110 realize a minimal inductance. In at least one exemplary
embodiment, the minimal inductance of each primary winding ranges
from approximately 40 microhenries (.mu.H) to approximately 90
.mu.H when the commutating device operates in the enabled mode.
If one or more current sensors 113 in FCL circuit 108 detect a
fault event (e.g., a short circuit that causes a current level to
exceed a current threshold), each of the commutating devices 114
initiates the disabled mode. The disabled mode opens (i.e.,
disconnects) the connection between the first and second ends of a
respective secondary winding 112. Accordingly, the second windings
112 are placed in series with one another instead of being isolated
to a closed loop such that the impedance of the secondary windings
112 changes. The disabled mode of each commutating device 114 may
be initiated to achieve an instantaneous change in the FCL circuit
108 impedance, or may be sequentially (i.e., staggered) initiated
to obtain a gradual change in the FCL circuit 108 impedance.
The changed impedance of the secondary windings 112 is realized by
a respective primary winding 110, thereby increasing the inductance
of the respective primary winding 110. In at least one embodiment,
the increased inductance of each primary winding ranges from
approximately 90 .mu.H to approximately 300 .mu.H. The increased
inductance of the primary windings 110 reduces the current level of
a fault current flowing through the respective PFN circuit 106.
Further, by placing each of the FCL circuits 108 included in the
PFN module 104 in series with one another, the fault energy (e.g.,
the increased energy caused by a fault event) may be equally
distributed among each PFN module 104 included in the PFN stack
102. Accordingly, the combination of the FCL circuit 108 and the
primary winding 110 of the PFN circuit 106 may protect the
switching assembly from irreversible damage during a fault event.
The change of inductance illustrated above allows, for example, the
maximum fault current to be less than 100 kA instead of 300 kA.
Referring now to FIG. 2, another exemplary embodiment of an
electromagnetic DC pulse power system 100 is illustrated, which
utilizes solid-state switches 116, such as a gate turn off (GTO)
thyristor switch 116, as the commutating device 114. Although GTO
thyristor switches 116 are illustrated in FIG. 2, other solid-state
switches 116 may be used including, but not limited to, an
integrated gate commutated thyristor (IGCT).
The electromagnetic DC pulse power system 100 of FIG. 2 also
includes an external protection circuit 118 and one or more current
detectors 120, 121 in the primary circuit. The external protection
circuit 118 includes a discrete resistor R.sub.X and a discrete
external inductor L.sub.X. The L.sub.X is configured to provide
additional inductance to the FCL circuit 108 such that the external
protection circuit may provide increased protection from high fault
current levels.
The current detectors may include a current transformer (CT) 120,
121 for example. The CT 120 is configured to detect a fault event
and control the solid-state switch 116 to initiate the disabled
mode. In at least one embodiment, a first CT 120 may be disposed
upstream from the primary winding 110 and a second CT 121 may be
disposed downstream from the primary winding 110 may to increase
the sensitivity for detecting a fault event. In particular, the
system of 120 and 121 allow for differential current monitoring of
the storage inductor and can pinpoint a line to ground fault within
the storage inductor primary winding 110.
For example, CT.sub.1 120 monitors the FCL loop current i.sub.1 and
outputs the measured current. A control system (not shown) may
analyze the measured i.sub.1. A terminal and/or internal fault
event may cause the primary current i.sub.1 to exceed a threshold
level (TH.sub.I). The CT.sub.1 120 triggers the group of GTO
thyristor switches 116 open. The GTO thyristor switches 116 may be
opened approximately 50 .mu.s after the CT 120 detects the fault.
When primary current exceeds the threshold, so does secondary
current i.sub.2 through the respective secondary windings 112. In
at least one exemplary embodiment, the secondary to primary winding
impedance ratio is 36:1 and may be achieved according to a turns
ratio N2:N1 of 6:1. Therefore, a 91 kA main fault current is
reflected as a 15.2 kA current in the FCL circuit 108. When the GTO
thyristor switches 116 opens, all secondary windings 112 may be
immediately placed in "series aiding". Accordingly, a total loop
inductance is provided, which equals a sum of the individual FCL
self-inductances and any external FCL loop inductance provided by
the external protection circuit 118 (i.e., R.sub.X and L.sub.X).
The secondary loop impedance (Z) may be represented as
Z=R.sub.X+j.omega.L.sub.X since fundamentally secondary loop
current contains an alternative current component at radian
frequency .omega. by virtue of the transformer coupling
present.
The FCL circuit 108 is configured to limit the maximum di/dt during
a fault event (e.g., a short circuit). As mentioned above, the
fault event is detected when the current (i.e., the primary
current) flowing through the PFN circuit 106 exceeds a current
threshold. One or more CTs 120 may be configured to determine if
di/dt exceeds a threshold (TH.sub.didt), and may control the
commutating device 114 to initiate the disabled mode. The disabled
mode maintains di/dt below TH.sub.didt and maintains critical
transient voltage (dv/dt) below a voltage rate threshold
(TH.sub.dvdt). As a result, the commuting device 114 can
effectively commutate the fault current. The TH.sub.didt may be,
for example, about 1000 A/.mu.s, and the dv/dt may be, for example,
2000 V/.mu.s.
Referring now to FIG. 3, another exemplary embodiment of a DC pulse
power system 100 is illustrated. The FCL circuit 108 of includes a
series solid-state commutating device 122 having a pair of GTO
thyristor switches 116. Each pair of GTO thyristor switches 116 may
be connected in series arrangement with respect to one another but
in parallel with device 123. When the FCL circuit 108 is enabled,
(i.e., when the commuting device 114 operates in the enabled mode)
the closed loop currents i.sub.21, i.sub.22, i.sub.23, and i.sub.24
are confined to a local loop of the secondary winding 112, as
discussed above. Each FCL circuit 108 may further include a metal
oxide varistor (MOV.sub.X) 123 connected in parallel with a
respective commutating device 122 and the complete circuit has
external impedance 120 included. The MOV.sub.X 123 may limit
over-voltage across the secondary winding 112 during a voltage
transient event.
The GTO thyristor switches 116 may have a reverse recovery charge
of approximately 6500 Amp-microseconds (A-.mu.s) to approximately
9500 (A-.mu.s), for example. In at least one embodiment, the GTO
thyristor switches 116 included in each PFN module 104 may have the
same Qrr. For example, the thyristor switches may each have a
blocking voltage of, for example, 6000 Volts such that each pair of
GTO thyristor switches 116 switches may have nearly equal voltage
distribution across them (by use of a divider network). The reverse
current rate of the GTO thyristor switches 116 increases, for
example when di/dt is -3.0 A-.mu.s, and Qrr increases to a value of
-14,000 A-.mu.s (see FIG. 4). This moderately high value of Qrr is
indicative of state of the art high power switching devices but
nevertheless allows rapid commutation of currents i.sub.21 to
i.sub.24 to proceed into the high impedance closed loop 108 of
current i.sub.5 and thereby transfer current into external
impedance element 124 to effect a sudden impedance change as
reflected in the primary windings of L.sub.x.
When the commutating devices 114 are triggered, a secondary loop
comprising the FCL circuits 108 (e.g., the four individual FCL
circuits 108) may be formed. In a secondary loop, the voltage on
the combined leakage inductances may be as high as 80 kV; this is
tolerable since it is dispersed across four distinct modules. This
defines the impulse voltage rating of the insulation system to
ground as 100 kV. Since the fault current must be rapidly
commutated, the commutating devices 114 (e.g., a pyrotechnic fuse,
GTO thyristor, etc.) must produce a back voltage (peak) of at least
3.0 per unit voltage of the PFN system input or charging
voltage.
Referring to FIG. 5, yet another embodiment of an electromagnetic
DC pulse power system 100 is illustrated. The electromagnetic DC
pulse power system 100 includes a PFN stack 102 connected to
dynamic load 124. The dynamic load 124 is shown as a combination of
an inductor L.sub.d and resistor R.sub.d with time-varying
component values. It is appreciated, however, that the dynamic load
124 may also consist of a combination of resistive, capacitive and
inductive elements with one or more of these elements having a time
dependent value. Moreover, the inductance L.sub.d in addition to
being time dependent may also be non-linear for its value of
inductance versus current.
The PFN stack 102 includes, for example, four PFN circuits 106 and
an FCL module 126. The PFN circuits 106 operate as described in
detail above. The FCL module 126 comprising one or more FCL units
128 that include a commutating device 114 and specialty transformer
T.sub.1-T.sub.4. When the commutating devices 114 are opened, an
isolated closed-loop FCL bus 130 is formed. Each FCL unit 128 is
magnetically connected to a respective PFN circuit 106. For
example, a first primary winding 110 of a first PFN circuit 106 is
connected to a first main winding 132 of the specialty transformer
of a first FCL unit 128. The inductor L.sub.1-L.sub.4 does not have
a secondary winding.
Each FCL unit 128 is arranged as a transformer assembly
T.sub.1-T.sub.4 that is separate from the storage inductor 110 of a
respective PFN circuit 106. The transformers T.sub.1-T.sub.4
include a main winding 132 and an auxiliary winding 134. Each FCL
unit 128 further includes a commutating device 114. The commutating
devices 114 may be a pyrotechnic fuse 114, for example. Although a
pyrotechnic fuse 114 is illustrated in FIG. 5, the commutating
device 114 may include, but is not limited to, a GTO thyristor or
IGCT device. Each pyrotechnic fuse 114 may be connected in parallel
with a MOV.sub.x 123. The auxiliary winding 134 of each transformer
T.sub.1-T.sub.4 may receive respective current i.sub.21, i.sub.22,
i.sub.23 and i.sub.24 and operates as discussed above. In addition,
the auxiliary windings 134 modulate the magnetic core flux from the
main windings 132 upon the transformer. Accordingly, the primary
currents i.sub.1-i.sub.4 provide greater control of the induction
and fault limiting action.
The electromagnetic DC pulse power system 100 of FIG. 5 further
includes a DC power supply 136. The DC power supply 136 may
generate a DC power, which drives and charges all the PFN circuits
106 either simultaneously or sequentially. It is understood that
switching devices S.sub.1 through S.sub.4, which form the DC power
pulse, may include multiple opening switches in series and/or in
parallel. The switching devices S.sub.1 through S.sub.4 have
solid-state switches in a preferred embodiment and may include a
thyristor, GTO thyristor or an IGCT. The electromagnetic DC pulse
power system 100 of FIG. 5 may also include clamp diodes
D.sub.1-D.sub.4, to prevent applying reverse voltage on the energy
storage capacitors C.sub.1 through C.sub.4.
Fault events may occur at various locations (e.g., 1-3) of a PFN
circuit 106. Each storage inductor 110 has a differential system of
dual current transformers (CT) 120, 121 configured to measure
current the level of current flowing through the storage inductor
110. For example, an input CT.sub.1 120 may monitor the input
current of a first inductor 110, and an output C.sub.T2 121 may
monitor the output current of the inductor 110. Accordingly, a
differential current protection system may be formed to protect the
PFN module 104 from potential ground faults realized. Further, the
differential current protection system segregates an internal PFN
fault from faults outside the PFN module 104 and helps isolate and
identify which PFN module 104 experiences a fault event including,
but not limited to, capacitor short circuit, diode open or short
circuit, thyristor open or short circuit or inductor open or short
circuit.
The transformers T1-T4 also provide a reference point at which a
transformer primary voltage (V.sub.t1-V.sub.t4) may be determined.
V.sub.t1-V.sub.t4 may be processed via a control system (not shown)
to determine which PFN module has higher or lower than predicted
operating characteristics. Based the comparisons of
V.sub.t1-V.sub.t4, the location of a PFN module 104 experiencing a
fault event may be ascertained. The fault limiter circuit of FIG. 5
can have all four commutating devices 114 triggered in a sequential
(i.e., staggered) manner such that a more gradual transient voltage
appearing on the transformer primaries at V.sub.t1 through
V.sub.t4.
Turning now to FIG. 6, another exemplary embodiment of the
electromagnetic DC pulse power system 100 is illustrated. Each PFN
module 104 includes a PFN circuit 106 and an FCL circuit 108 as
described in detail above. The FCL circuit 108 according to the
exemplary embodiment of FIG. 6 includes one or more electrically
isolated windings 138, one or more isolated switching devices 140,
and an auto-transformer device 142 with windings 138, 139, 141.
Accordingly, a two-stage fault-limiting system is provided, whereby
the impedance or inductance of winding 139 is increased upon
opening action of isolated switching devices 140, shown arranged as
a bilateral switch. The main switching devices 111 have shunt
connected surge protective devices 109 and an opening switcher, of
either the solid-state devices or pyrotechnic opening devices.
Fault currents can originate at the load end circuit and be driven
"backwards" to cause very high current in the primary current
i.sub.1 and i.sub.2 path if, for example, the capacitor C.sub.1 or
C.sub.2 has an internal short circuit. Alternately, if a short
circuit occurs at a load external to the electromagnetic DC pulse
power system 100, the capacitor C.sub.1 and/or C.sub.2 may allow
for a "forward" high surge current, which may damage other
components. The electromagnetic DC pulse power system 100 of FIG. 6
may prevent damage caused by both forward and reverse type of fault
currents.
More specifically, the isolated windings 138 are connected in
series with an isolated switching device 140, for example a
bilateral thyristor pair 140. In a normal state (i.e., a
non-fault), the isolated switching device 140 are closed to
maintain reflected inductance in the primary windings 139 as low as
possible. The primary winding 139 turns ratio of P.sub.1:S.sub.3
and P.sub.2:S.sub.4 can be arbitrary and allows the auxiliary
circuit with current i.sub.z to be implemented as a high voltage,
low current circuit. Devices PF.sub.1 and PF.sub.2 are pyrotechnic
trigger fuses connected to auto-transformer windings 139 and 141
and are triggered to open if the fault current in circuit 106
exceeds an upper threshold. For lower level faults, devices 140 are
triggered to open. The opening of PF.sub.1 and PF.sub.2 generates
an impedance change as understood by those ordinarily skilled in
the art.
The auto-transformer device 142 is configured to reduce the
magnetic materials and overall volume of the inductor/transformer
combination in comparison to previous embodiments shown in this
application. Commutating devices 140 and current sensors 143 detect
a high rate of rise (di/dt) of current in either i.sub.1 or i.sub.2
paths. Accordingly, the effective series inductance of the
auto-transformer windings 139 is increased, and the fault current
flowing from the main capacitor to the bus (or vice versa) is
reduced. After a period of time after triggering the main
commutating devices 111 to open, for example about 2-5 ms, a
control signal may be output to either PF.sub.1/PF.sub.2 or to
switch 140 or both to open simultaneously. Accordingly, an
additional protection operation may be provided that increases the
effective inductance realized by the primary windings P.sub.1 and
P.sub.2. The interconnected circuitry of the i.sub.z current path
helps balance the action of the fault limiting among all primaries
windings to be magnetically coupled through the auto-transformer.
In an exemplary embodiment, the number of turns for the primary
windings (P.sub.1, P.sub.2) may be N.sub.1=2, the number of turns
for the secondary windings (S1, S2) may be N.sub.2=4, and the
number of turns for the auxiliary windings (S.sub.3, S.sub.4) may
be N.sub.x=8.
Due to the high power output generated by one or more of the PFN
modules 104, the electromagnetic DC pulse power system 100 may
further include a cooling system. The cooling system may cool one
or more of the PFN modules 104, thereby improving overall operation
and reliability of the electromagnetic DC pulse power system 100 as
discussed in greater detail below. The cooling system also
functions to remove a portion of the PFN fault energy and
transferred this waste heat to an external media.
Referring to FIG. 7, a temperature regulating pulse forming network
(TRPFN) module 200 is illustrated. The TRPFN module 200 includes an
inner housing 202, a primary winding 204, a secondary winding 206,
an inner magnetic core 208, a cooling system 210, and an outer
housing 212. The inner housing 202 supports the primary winding
204, the secondary winding 206, the inner magnetic core 208, and
the cooling system 210. For example, the inner housing 202 may
define individual chambers containing the primary winding 204, the
secondary winding 206, and the inner magnetic core 208. In at least
one embodiment, the TRPFN module 200 may extend axially along a
Y-axis to define a height (H) and radially about an axis (A) to
define a radius (R).
The inner magnetic core 208 may be disposed in a first chamber 214
of the inner housing 202. The inner magnetic core 208 may be formed
from a non-saturating material that is wound circumferentially
about the axis (A) within the first chamber 214, and is configured
to induce an electromagnetic field when current flows through the
primary winding 204 and/or secondary winding 206. The
non-saturating material includes, but is not limited to, high
cobalt high permeability steels. In at least one exemplary
embodiment, the inner magnetic core 208 may include, for example,
three segmented magnetic sub-cores 216. The sub-cores 216 may be
toroidally wound about the axis (A), and may have a thickness of
approximately 10 millimeters (mm), for examples. Each sub-core 216
may be magnetically separated from one another by a core isolator
218 to reduce eddy current losses and reduce the inner magnetic
field. The core isolator 218 may include, for example, an air gap
218 or a solid dielectric material such as polypropylene, nylon or
fiberglass epoxy.
The secondary winding 206 may be contained in a second temperature
regulating chamber 220 of the inner housing 202. The second chamber
220 may be disposed adjacent to the first chamber 214 such that the
secondary winding 206 may control the inductance of the primary
winding 204 as discussed in detail above. The secondary winding 206
may be formed as one or more winding layers 222. The secondary
winding 206 in each winding layer 222 is wound to form a number of
secondary turns. The number of turns of the secondary winding 206
is greater than the number of turns of the primary winding 204.
Accordingly, the impedance of the secondary winding 206 is greater
than the impedance of the primary winding 204. Referring FIG. 7,
for example, the secondary winding 206 may be formed as two winding
layers 222 having a total of 72 turns. The number of layers and/or
the number of turns however, are not limited thereto. The secondary
winding 206 may also include secondary terminals 223 to connect the
secondary winding 206 to a FCL circuit 108.
The primary winding 206 may be disposed in a third chamber 224 that
surrounds the second chamber 220. In at least one exemplary
embodiment, the primary winding 204 may include a plurality of
turns, for example 24 turns, in series with one another. More
specifically, the third chamber 224 may include a plurality of
sub-chambers 226 extending along the Y-direction (i.e., the height)
of the inner housing 202. The primary winding 204 may also include
primary terminals 227 to connect the primary winding 204 to a PFN
circuit 106 as shown in FIG. 2.
The sub-chambers 226 are disposed next to one another and are
configured to receive a primary winding conductor 228 of the
primary winding 204. The primary winding 204 may be toroidally
wound in the sub-chambers 226 such that primary winding 204
portions are spaced apart from one another primarily to increase
dielectric strength. The number of primary winding conductors 228
in each sub-chamber 226 may be equal. Referring still to FIG. 7,
the number of turns formed by the primary winding 204 may comprise,
for example, eight turns in series per layer. A combination of the
primary winding conductors 228 contained in a single sub-chamber
226 may define a primary layer that extends in the height-direction
(H) of the TRPFN module 200. Since the exemplary embodiment of FIG.
7 illustrates eight turns per sub-chamber 226, the primary winding
204 includes a total of twenty-four winding turns. Although a total
of three primary winding layers and three sub-chambers 226 are
illustrated in FIG. 7, the number of sub-chambers 226 and/or the
number of primary winding portions in each sub-chamber 226 are not
limited thereto.
The primary winding 204 may be formed from various electrically
conductive materials including, for example, copper. The primary
winding 204 may also be formed of various cross-sectional shapes
including, but not limited to, circular-shaped and square-shaped.
In at least one embodiment, a first primary winding conductors 228
may be wound in a first sub-chamber 226 to define a first diameter
ranging from approximately 8 inches to approximately 9 inches, for
example. A second primary winding conductors 228 may be wound
within a second sub-chamber 226 to define a second diameter ranging
from approximately 9 inches to approximately 10 inches, for
example. A third primary winding portion 228 may be wound within a
third sub-chamber 226 to define a third diameter ranging from
approximately 10 inches to approximately 11 inches, for example. As
further illustrated in FIG. 7, the primary winding 204 is separated
from the secondary winding 206 by a distance of approximately 2
inches, which generates a loose coupling between the primary
windings 204 and secondary winding 206.
The cooling system 210 is configured to cool the primary winding
204 and/or the secondary winding 206. A portion of the PFN fault
energy is dissipated in the primary and secondary windings which is
subsequently transferred to an external heat exchanger or external
media. In at least one exemplary embodiment, the cooling system 210
includes a primary cooling system 230 that cools the primary
winding 204 and a secondary cooling system 232 that cools the
secondary winding 206. The cooling system 230 may be direct
liquid-injection and the cooling system 232 may be indirect
conduction mechanism. The primary cooling system 230 may be
configured differently than the secondary cooling system 232, as
discussed in greater detail below.
The primary cooling mechanism is formed as an integrated cooling
system 230 that is integrated with the primary winding 204. In at
least one exemplary embodiment illustrated in FIG. 7, the primary
cooling system 230 includes a channel 234 formed through
approximately the center of the primary winding 204. A primary
coolant may be flowed or injected through the channel 234, thereby
providing an internal cooling feature that cools the primary
winding 204 from within. The primary coolant may include, but is
not limited to, water, mineral oil and hydrofluoroether (HFE), or
fluoroketones for example.
The primary cooling system 230 may be formed as a single phase
cooling system or a two-phase liquid cooling system. The two-phase
cooling system may include a two-phase liquid and gas cooling
system. In another embodiment, the primary cooling system 230 may
include a chill plate and/or an auxiliary cooling device 231. The
chill plate and/or an external cooling device may be in thermal
communication with the primary winding 204 to reduce the heat
thereof.
The secondary cooling system 232 may be formed in combination with
the second chamber 220. More specifically, in addition to
containing the secondary winding 206, the second chamber 220 may be
configured to contain a secondary coolant that surrounds the
secondary winding 206. The secondary coolant contained in the
second chamber 220 may include a non-flammable, high-dielectric,
high electrical resistivity fluid such as, for example, HFE.
The secondary cooling system 232 may be formed as a two-phase
liquid and gas cooling system. In another embodiment, the secondary
cooling system 232 may include a chill plate and/or an external
cooling device. The chill plate and/or an external cooling device
may be in thermal communication with the secondary winding 206 to
reduce the heat thereof. For example, the auxiliary cooling device
232 may be a liquid coaxial-shaped annulus with internal cooling
channels and may include one or more cooling lines 236 interposed
between one or more wound portions of the secondary winding 206.
The cooling lines 236 may flow a coolant therethrough, thereby
cooling the secondary winding 206. The cooling system transfer heat
from the primary and secondary windings to an external heat
exchanger (not shown), represents a portion of the subject PFN
fault energy. Similarly, the portion of PFN fault energy dissipated
as heat in resistor Rx in element 120 FIG. 3 must also be removed
from the described apparatus and transferred to an external heat
exchanger or external media.
The outer housing 212 surrounds the inner housing 202. In at least
one exemplary embodiment, the outer housing 212 is formed from a
highly conductive material and is disposed against the inner
housing 202. The highly conductive material of the outer housing
212 may include, but is not limited to, aluminum or brass.
Accordingly, the outer housing 212 may be configured as an
electromagnetic shield that inhibits electromagnetic waves emitted
from the TRPFN module 200.
Another embodiment of a TRPFN module 200' is illustrated in FIG. 8.
The TRPFN module 200' is similar to the TRPFN module 200 of FIG. 7;
however, the number of primary winding portions 228 per layer
included in the sub-chambers 226 is unequal to taper the internal
magnetic field and this has a higher primary to secondary coupling
coefficient than in FIG. 7. In addition, the secondary winding 206
may be formed as four layers instead of the two layers illustrated
in FIG. 7. Accordingly, the turns ratio (i.e., N.sub.2:N.sub.1) of
the primary windings 204 with respect to secondary windings 206 may
be, for example, 6 secondary winding turns for a single primary
winding turn (i.e., 6:1). Further, the radial separation between
primary winding 204 and the secondary winding 206 may be decreased
to approximately 0.5 inch. Magnetic isolators 225 are shown in both
the outer steel housing and in the inner core. The magnetic
isolators 225 may control the saturation of the overall magnetic
circuit. The saturation may be controlled due to a quasi-linear
inductance versus current characteristic that is obtained at high
fault currents. The smaller radial separation between windings of
FIG. 8 forms a higher magnetic coupling between the primary winding
204 and secondary winding 206 instead of the loose coupling
illustrated in FIG. 7. Also the time constant of the embodiment of
FIG. 8 is shorter than FIG. 7 since the inner magnetic cores use
laminated electrical high permeability steel of smaller total
volume.
Turning now to FIG. 9, a method of protecting an electromagnetic DC
pulse power system in response to a fault event is illustrated. The
method begins at operation 500 and proceeds to operation 502 to
deliver DC power to a pulse forming network (PFN) circuit. At
operation 504, a fault current limiting (FCL) circuit is
magnetically coupled to the PFN circuit. At operation 506, a
determination as to whether a fault event exists. If the fault
event does not exist, a shaped DC pulse is output from the
electromagnetic DC pulse power system at operation 508 and the
method ends at operation 510.
If the fault event is determined at operation 506, the secondary
impedance of the FCL circuit is regulated to control a primary
impedance of the PFN circuit at operation 512. At operation 514, a
portion of the fault energy from the PFN circuit is transferred to
the FCL circuit in response to controlling the impedance of the PFN
circuit, thereby lowering the prospective primary fault current and
fault power and the method ends at operation 510.
Turning now to FIG. 10, a method of protecting an electromagnetic
DC pulse power system including an auto-transformer is illustrated
according to another exemplary embodiment of the disclosure. The
method begins at operation 600, and proceeds to operation 602 to
deliver DC power to a pulse forming network (PFN) circuit to charge
an energy storage capacitor. At operation 604, a fault current
limiting (FCL) circuit is magnetically coupled to the PFN circuit.
At operation 606, a determination as to whether a fault event
exists. If the fault event does not exist, a shaped DC pulse is
output from the electromagnetic DC pulse power system at operation
608 and the method ends at operation 610.
If the fault event is determined at operation 606, a magnitude of
the fault event is determined. For example, if a fault magnitude
may be detected if a source current exceeds a first current
threshold. If the source current exceeds a second threshold, then
the magnitude of the fault event may be determined as a large fault
event. If the fault magnitude is not large, a first fault-limiting
operation is initiated at operation 614. More specifically, at
operation 614, a secondary impedance of the FCL circuit is
regulated to control the primary impedance of a PFN circuit. At
operation 616, fault energy is transferred from the PFN circuit to
an i.sub.x and i.sub.y loop. Accordingly, the fault current and
fault power is reduced. At operation 618, a shaped DC pulse having
a reduced fault energy is output from the DC pulse power
system.
If the fault magnitude determined at operation 612 is large, a
second fault-limiting operation is initiated at operation 620. More
specifically, a reflected impedance of the auto-transformer. For
example, pyrotechnic trigger fuses may be connected to windings of
the auto-transformer. If the fault magnitude is determined as large
at operation 612, the pyrotechnic trigger fuses are triggered to
open. At operation 622, increase reflected impedance in the primary
winding of the PFN circuit after expiration of time delay. At
operation 624, the fault current in the PFN is reduced in response
to the increased reflected impedance in the primary winding, and
the method ends at operation 610.
The corresponding structures, materials, acts, and equivalents of
all means or step plus function elements in the claims below are
intended to include any structure, material, or act for performing
the function in combination with other claimed elements as
specifically claimed. The description of various embodiments
described herein has been presented for purposes of illustration
and description, but is not intended to be exhaustive or limited to
the invention in the form disclosed. Many modifications and
variations will be apparent to those of ordinary skill in the art
without departing from the scope and spirit of the invention. The
embodiments were chosen and described in order to best explain
various principles and features of the practical application, and
to enable others of ordinary skill in the art to understand the
invention for various embodiments with various modifications as are
suited to the particular use contemplated.
While the various embodiments have been described, it will be
understood that those skilled in the art, both now and in the
future, may make various modifications which fall within the scope
of the following claims. These claims should be construed to
maintain the proper protection for the invention first
described.
* * * * *